1. Field of the Invention
The present invention relates to optical pickup units and optical disk drive units, and more particularly to an optical pickup unit that emits light beams of different wavelengths to the recording surfaces of different types of information recording media and receives reflected lights therefrom, and an optical disk drive unit employing such an optical pickup unit.
2. Description of the Related Art
An optical pickup unit that is designed for both a DVD-type optical recording medium using a 650 nm wavelength xcex1, such as a digital video disk, and a CD-type optical recording medium using a 780 nm wavelength xcex2, such as a compact disk, has been put to practical use. Such an optical pickup unit employs, as its light source, a semiconductor laser chip manufactured as a xe2x80x9cmonolithic chipxe2x80x9d emitting light beams of the respective wavelengths xcex1 and xcex2 (hereinafter referred to as a xe2x80x9cdual-wavelength monolithic chipxe2x80x9d), or a package of different semiconductor laser chips emitting laser beams of the respective wavelengths xcex1 and xcex2 in order to use in common the optical system in the optical path from the light source to the optical recording medium.
In the dual-wavelength monolithic chip, the respective light-emitting parts can be positioned close to each other. It is not easy, however, to position the light-emitting parts relative to each other with accuracy, thus making it difficult to increase the manufacturing yield of the chip. Further, in the case of containing the light source and a xe2x80x9clight-receiving element receiving a returning beamxe2x80x9d in one package, heat from the dual-wavelength monolithic chip makes it difficult for the light-receiving element to operate at high speed if the light-emitting parts are positioned with a small distance between each other. Therefore, it is difficult to apply the dual-wavelength monolithic chip in the case of recording information on an optical recording medium or reproducing information therefrom at high speed.
On the other hand, in the case of containing semiconductor laser chips of different light emission wavelengths in a single package, the semiconductor laser chips having desired outputs for the respective, wavelengths can be used. Therefore, the best semiconductor laser chip can be used in accordance with the specifications of the optical disk drive unit. Accordingly, a high-speed optical disk drive unit can be realized at low cost.
However, since the semiconductor laser chips are individually mounted in this type of light source, an error is caused in the mounting of the semiconductor laser chips. Therefore, the accuracy of spacing the two light-emitting parts is likely to be decreased. Such a decrease in the spacing accuracy is likely to incur a decrease in the signal detection accuracy of the light-receiving element.
Some optical recording media have substrates of high birefringence. Therefore, in the case of using an optical disk drive unit in common between a plurality of types of optical recording media using different wavelengths, there is the problem of xe2x80x9cdeterioration in detection signalsxe2x80x9d, which is a variation caused in detection signals such as reproduction, focus, and tracking signals by the effect of birefringence when information is recorded on or reproduced from an optical recording medium having a substrate of high birefringence.
An optical disk unit uses an information recording medium such as an optical disk. The optical disk unit records information on the information recording medium by focusing a laser beam onto its recording surface on which a spiral track or concentric tracks are formed. The optical disk unit reproduces information from the optical recording medium based on a reflected light from its recording surface. The optical disk unit includes an optical pickup unit as a device for emitting the laser beam into a beam spot on the recording surface of the optical recording medium and receiving the reflected light therefrom.
Normally, the optical pickup unit includes an objective lens guiding a light beam emitted from a light source to the recording surface of the optical recording medium, an optical system guiding the light beam reflected from the recording surface as a returning beam to a predetermined light-receiving position, and a light-receiving element provided at the predetermined light-receiving position. The light-receiving element outputs a signal including not only the reproduced information of data recorded on the recording surface, but also information necessary for controlling the optical pickup unit itself and the position of the objective lens (servo information).
In order to correctly record data at a predetermined position on the recording surface or correctly reproduce data recorded at a predetermined position on the recording surface, it is necessary to form the laser spot at the predetermined position with accuracy. For this purpose, it is required to detect the position at which the laser spot is formed. A variety of methods of detecting a laser spot formation position on the recording surface by using the returning beam reflected therefrom have been proposed, and some of the methods have been put to practice.
The methods of detecting a laser spot formation position on the recording surface can be classified roughly into the following two types. One type uses a returning beam from one beam spot formed on the recording surface. A method of this type is called a one-beam method. The other type uses returning beams from three beam spots formed on the recording surface. A method of this type is called a three-beam method. In the case of using the three-beam method, the light beam emitted from the light source is split into three beams to form the three beam spots on the recording surface.
Of one-beam methods, a so-called push-pull method and a phase difference method are commonly used, while a so-called three-spot method and a differential push-pull method are commonly used three-beam methods.
The push-pull method divides the returning beam into two parts in a direction corresponding to the tangential direction of the track and detects the deviation of a beam spot position (a so-called tracking error signal) from the difference between the amounts of light of the two parts.
The phase difference method detects a tracking error signal based on the rotational change of the intensity pattern of the returning beam. That is, the returning beam is detected by a light-receiving elements divided into four parts, and obtains a phase lead and a phase lag based on signals each representing the sum of the amounts of light received by a corresponding pair of light-receiving element parts positioned diagonally to each other. Thereby, the tracking error signal is obtained.
According to the three-spot method, the light beam emitted from the light source is divided into one main (primary) beam and two sub (secondary) beams. The light beam is emitted so that the main beam and the sub beams are focused onto the recording surface into respective laser spots that are equally spaced a quarter track pitch apart in the tracking directions, which are directions perpendicular to the tangential directions of the track. The returning beams of the two sub beams reflected back from the recording surface are received by their respective light-receiving elements, so that a tracking error signal is detected from the difference between the amounts of light received by the two light-receiving elements.
According to the differential push-pull method, the light beam emitted from the light source is divided into one main beam and two sub beams. The light beam is emitted so that the main beam and the sub beams are focused onto the recording surface into respective laser spots that are equally spaced a half track pitch apart. The returning beams of the main beam and the sub beams reflected back from the recording surface are received by three respective light-receiving elements each divided into two parts. Then, a push-pull signal is obtained in each of the light-receiving elements, and a tracking error signal is obtained from a signal representing the difference between the push-pull signal of the main beam and a signal representing the sum of the push-pull signals of the two sub beams.
In the optical disk unit, when the objective lens is moved in-the tracking directions to form the beam spot accurately at the predetermined position on the recording surface, that is, when so-called tracking control is performed, the tracking error signal is detected as described above from the output signal of each light-receiving element, and is fed back to control the position of the objective lens in the tracking directions.
In recent years, DVDs (Digital Versatile Disks) much larger in recording capacity than CDs (Compact Disks) have been widely used. A laser beam of a 780 nm wavelength is used for CD recording and reproduction, and a laser beam of a 650 nm wavelength is used for DVD recording and reproduction. Therefore, conventionally, an optical disk unit for the CDs and an optical disk unit for the DVDs have been used as independent peripheral devices for an information apparatus such as a personal computer.
As the information apparatus has become smaller in size and lighter in weight, demand for an optical disk unit that can access both DVDs and CDs has increased. In this case, in order to accommodate both DVDs and CDs, the optical pickup unit is required to include, as light sources, a semiconductor laser emitting the laser beam of 650 nm wavelength (hereinafter this semiconductor laser is also referred to as xe2x80x9ca DVD light sourcexe2x80x9d) and a semiconductor laser emitting the laser beam of 780 nm wavelength (hereinafter this semiconductor laser is also referred to as xe2x80x9ca CD light sourcexe2x80x9d). The optical disk unit is required to further include an optical system for detecting each of the laser beams. However, if optical systems are separately provided for the 650 nm and 780 nm wavelengths, there is the disadvantage that the optical pickup unit becomes larger in size. Hereinafter, such an optical pickup unit having light sources of two different wavelengths is also referred to as a xe2x80x9ctwo-wavelength optical pickup unit.xe2x80x9d
For instance, a small-scale two-wavelength optical pickup unit that can perform both DVD and CD reproduction by using a two-wavelength integrated laser diode (TWIN-LD) into which the DVD and CD light sources are monolithically integrated is disclosed in xe2x80x9cDevelopment of 7.3 mm Height DVD Optical Pickup Using TWIN-LDxe2x80x9d described in pp. 6 to 9 of the preliminary reports of the 7th Microoptics Conference held in July 1999 (hereinafter referred to as xe2x80x9cfirst prior artxe2x80x9d). This optical pickup unit includes an LD-PD assembly formed by packaging the TWIN-LD and a light-receiving element (a photodiode (PD)) in a housing, a grating element that divides only a light beam emitted from the CD light source into three beams, and one hologram that guides each of the returning beams of the respective wavelength to the light-receiving element. A tracking error signal is detected by the above-described phase difference method for the DVDs and by the above-described three-spot method for the CDs.
Japanese Laid-Open Patent Application No. 2001-216677 (hereinafter referred to as xe2x80x9csecond prior artxe2x80x9d) discloses a low-cost two-wavelength optical pickup unit that can perform both DVD and CD reproduction by using the DVD and CD light sources, an information reproduction apparatus using the same, and an information recording and reproduction apparatus using the same. In the optical pickup unit of the second prior art, each of the light beams emitted from the DVD and CD light sources is divided into three beams by using two diffraction gratings with wavelength selectivity. Each of the returning light beams of the respective wavelengths is diffracted by one diffraction grating for signal detection, so that the diffracted light is received by a light-receiving element. A tracking error signal is detected by the above-described phase difference method for DVD-ROMs, by the above-described differential push-pull method for DVD-RAMS, and by the above-described three-spot method for the CDs.
Japanese Laid-Open Patent Application No. 2000-76689 (hereinafter referred to as xe2x80x9cthird prior artxe2x80x9d) discloses a two-wavelength optical pickup unit that can perform both DVD and CD recording and reproduction by using the DVD and CD light sources. In the optical pickup unit of the third prior art, at least one of the light beams emitted from the DVD and CD light sources is divided into three beams by one diffraction grating for division. The optical pickup unit of the third prior art includes a first hologram element optimized for the light beam of 650 nm wavelength and a second hologram element optimized for the light beam of 780 nm wavelength, so that the light beams of 650 nm and 780 nm wavelengths are diffracted by the first and second hologram elements, respectively, toward a light-receiving element. A tacking error signal is detected by the three-spot method With respect to the light beam divided into the three beams and by the push-pull method with respect to the single light beam (not divided into three beams).
As information to be recorded on optical recording media is increasingly diversified, attempts have been made to perform information recording and reproduction at higher rates. Particularly, the DVDs, which have high recording density, need tracking control at higher speed with more accuracy if the recording rate is to increase.
In the case of the first prior art, this poses no problem if the optical pickup unit is used only for reproduction. If the optical pickup unit is also used for recording, however, recording may not be performed with accuracy, particularly, in the case of DVD recording. This is because a tracking error signal is detected by the phase difference method in the case of the DVDs so that an offset component may be added to tracking error information output from the light-receiving element because of the deviation of the optical axis of the returning beam caused by the shift of the objective lens. Further, since the light sources are integrated monolithically, it is difficult to increase the output of each light source, for instance, to a 100 mW level. Therefore, the optical pickup unit of the first prior art has difficulty in operating at high recording and reproduction rates.
In the second prior art, the tracking error signal is detected by the three-spot method for the CDs. Therefore, an offset component may be added to the output signal of the light-receiving element due to an inclination of the optical disk in the tracking directions, thereby preventing accurate recording. Further, the light sources are provided in the same package, but the light-receiving element is provided not in the package but in another position. Therefore, the optical pickup unit is not satisfactorily downsized.
In the third prior art, each of the light beams emitted from the respective light sources is divided into three beams by the single diffraction grating for division. This makes it difficult to focus both 650 nm and 780 nm light beams accurately on predetermined positions on the recording surfaces, of DVD and CD disks, respectively. Further, the tracking error signal is detected by the three-spot method with respect to each light beam divided into three beams. Therefore, an offset component may be added to the output signal of the light-receiving element due to an inclination of the optical disk in the tracking directions, thus preventing accurate recording.
Accordingly, it is a general object of the present invention to provide an optical pickup unit in which the above-described disadvantages are eliminated and an optical disk drive unit employing such an optical pickup unit.
A more specific object of the present invention is to provide an optical pickup unit that can perform excellent signal detection with respect to each of the different light-emission wavelengths of a plurality of semiconductor laser chips provided in a package as a light source even if the light-emitting parts of the semiconductor laser chips are positioned with less accuracy, can effectively reduce the deterioration of a detected signal due to the influence of the birefringence of the substrate of an optical recording medium, can increase the usability of light, and can operate at high speed.
Another more specific object of the present invention is to provide an optical pickup unit that can be used for a plurality of types of information recording media and can accurately obtain tracking error information without an increase in size and cost.
Yet another more specific object of the present invention is to provide an optical disk drive unit employing such an optical pickup unit so as to be usable for a plurality of types of information recording media and perform accurate recording with stability.
The above objects of the present invention are achieved by an optical pickup unit for performing at least one of information recording, reproduction, and erasure, the optical pickup unit including: a light source of a plurality of semiconductor laser chips of different light-emission wavelengths; a plurality of holograms provided to respective substrates, the holograms being placed between the light source and an optical recording medium, the holograms including at least one non-polarization hologram having a substantially uniform diffraction efficiency irrespective of a direction of polarization of incident light and at least one polarization hologram having a diffraction efficiency varying depending on a direction of polarization of incident light; and a wave plate provided closer to the optical recording medium than the polarization hologram is, wherein a light beam emitted from a selected one of the semiconductor laser chips passes through the holograms to be focused onto a recording surface of the optical recording medium and reflected therefrom as a returning beam, the returning beam is diffracted by a corresponding one of the holograms so that a diffracted light of the returning beam is received by a light-receiving element, and the wave plate turns a direction of polarization of the returning beam to a different direction from that of the light beam emitted from the selected one of the semiconductor laser chips.
In the above-described optical pickup unit, the positioning accuracy of the light-emission parts of the semiconductor laser chips in the light source may be lower than that of the two-wavelength monolithic chip. However, by individually adjusting the holograms, focus and tracking control can be performed with respect to each of the different wavelengths, so that an excellent signal with a reduced offset can be detected.
Further, by including at least one non-polarization hologram and at least one polarization hologram, a returning beam of a wavelength used for an optical disk (optical recording medium) whose substrate has large birefringence is diffracted by the non-polarization hologram to be detected so that a variation caused by the birefringence of the disk substrate may be reduced, and a returning beam of another wavelength used for another type of optical disk is diffracted by the polarization hologram to be detected so that the light beam emitted from the light source can be focused onto the optical disk with increased power and the amount of light of the returning beam received by the light-receiving element can be increased. Thereby, the optical pickup unit of the present invention can be used for high-speed recording and reproduction.
The above objects of the present invention are also achieved by an optical disk drive unit for performing at least one of information recording, reproduction, and erasure with respect to an optical recording medium selected from disk-like optical recording media of a plurality of types using different wavelengths, the optical disk drive unit including an optical pickup unit including: a light source of a plurality of semiconductor laser chips of different light-emission wavelengths; a plurality of holograms provided to respective substrates, the holograms being placed between the light source and the optical recording medium, the holograms including at least one non-polarization hologram having a substantially uniform diffraction efficiency irrespective of a direction of polarization of incident light and at least one polarization hologram having a diffraction efficiency varying depending on a direction of polarization of incident light; and a wave plate provided closer to the optical recording medium than the polarization hologram is, wherein a light beam emitted from a selected one of the semiconductor laser chips passes through the holograms to be focused onto a recording surface of the optical recording medium and reflected therefrom as a returning beam, the returning beam is diffracted by a corresponding one of the holograms so that a diffracted light of the returning beam is received by a light-receiving element, and the wave plate turns a direction of polarization of the returning beam to a different direction from that of the light beam emitted from the selected one of the semiconductor laser chips.
The above objects of the present invention are also achieved by an optical pickup unit including: a plurality of light sources emitting light beams of different wavelengths; an optical system including: an objective lens focusing any of the light beams onto a recording surface of an information recording medium attached to the optical pickup unit; a plurality of grating elements provided between the light sources and the objective lens, the grating elements corresponding to the different wavelengths, respectively, and dividing the light beams of the corresponding wavelengths traveling toward the objective lens each into a plurality of beams including a 0th-order light and diffracted lights; and a plurality of holograms corresponding to the different wavelengths, respectively, and guiding to a predetermined position the light beams of the corresponding wavelengths reflected from the recording surface; and a photodetector provided at the predetermined position, wherein one of the light sources which one emits a light beam of a wavelength corresponding to a type of the attached information recording medium is selected, and the light beam emitted from the selected one of the light sources is focused onto the recording surface and reflected therefrom to be received by the photodetector.
According to the above-described optical pickup unit, the light beam emitted from each of the light sources is divided into a plurality of beams including a 0th-order light and diffracted lights by the corresponding one of the grating elements so that the beams are focused onto the recording surface of the information recording medium whose type corresponds to the wavelength of the emitted light beam. Thereby, the beam spots of the respective beams are formed on the recording surface. Each of the beams is reflected from the recording surface as a plurality of returning beams to be incident on the corresponding one of the holograms. The returning beams are diffracted by the corresponding one of the holograms so as to be detected by the photodetector provided at a predetermined position. That is, the above-described optical pickup unit includes the grating elements and the holograms corresponding to the wavelengths of the light beams emitted from the respective light sources. Therefore, tracking error information can be obtained with accuracy with respect to a plurality of types of information recording media. Further, grating elements and holograms are inexpensive small optical elements, thus preventing an increase in the size and cost of the optical pickup unit.
The above objects of the present invention are further achieved by an optical disk unit performing information recording by emitting light onto a recording surface of an information recording medium, the optical disk unit including: an optical pickup unit including: a plurality of light sources emitting light beams of different wavelengths; an optical system including: an objective lens focusing any of the light beams onto the recording surface of the information recording medium; a plurality of grating elements provided between the light sources and the objective lens, the grating elements corresponding to the different wavelengths, respectively, and dividing the light beams of the corresponding wavelengths traveling toward the objective lens each into a plurality of beams including a 0th-order light and diffracted lights; and a plurality of holograms corresponding to the different wavelengths, respectively, and guiding to a predetermined position the light beams of the corresponding wavelengths reflected from the recording surface; and a photodetector provided at the predetermined position; and a processing device performing the information recording by using an output signal of the optical pickup unit, wherein one of the light sources which one emits a light beam of a wavelength corresponding to a type of the attached information recording medium is selected, and the light beam emitted from the selected one of the light sources is focused onto the recording surface and reflected therefrom to be received by the photodetector.
According to the above-described optical disk unit, information recording can be performed on information recording media of a plurality of types with accuracy and stability.